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Thursday, 12 January 2017

A Gauss meter with a Hall Effect probe is commonly used to measure the tangential field strength on the surface of the part. As discussed in some detail on the measuring magnetic fields page the Hall Effect is the transverse electric field created in a conductor when placed in a magnetic field. Gauss meters, also called Tesla meters, are used to measure the strength of a field tangential to the surface of the magnetized test object. The meters measure the intensity of the field in the air adjacent to the component when a magnetic field is applied.
The advantages of Hall effect devices are: they provide a quantitative measure of the strength of magnetizing force tangential to the surface of a test piece, they can be used for measurement of residual magnetic fields, and they can be used repetitively.
Their main disadvantages are that they must be periodically calibrated and they cannot be used to establish the balance of fields in multidirectional applications.

Sunday, 25 December 2016

The 400 watt metal halide bulbs or "super light" can be found in some facilities. This super bright will provide adequate lighting over an area of up to ten times that covered by the 100 watt bulb. Due to their high intensity, excessive light reflecting from the surface of a component is a concern. Moving the light a greater distance from the inspection area will generally reduce this glare. Another type of high intensity light available is the micro - discharge light. This particular light produces up to ten times the amount of UV light conventional lights produce. Reading of up to 60,000 uW/cm2 at 15 inches can be achieved.
Determining whether a magnetic field is of adequate strength and in the proper direction is critical when performing magnetic particle testing. As discussed previously, knowing the direction of the field is important because the field should be as close to perpendicular to the defect as possible and no more than 45degrees from normal. Being able to evaluate the field direction and strength is especially important when inspecting with a multidirectional machine, because when the fields are not balanced property, a vector field will be produced that may not detect some defects.
There is actually no easy -to-apply method that permits an exact measurement of field intensity at a given point with in a material. In order to measure the field strength, it is necessary to intercept the flux lines. This is impossible without cutting into the material and cutting the material would immediately change the field within the part. However, cutting a small slot or hole into the material and measuring the leakage field that crosses the air gap with a Gauss meter is probably the best way to get an estimate of the actual field strength within a part. Nevertheless, there are a number of tools and methods available that are used to determine the presence and direction of the field surrounding a component.

Wednesday, 30 November 2016

Ultraviolet light or "black light" is light in the 1000 to 4000 Angstroms (100 to 400nm) wavelength range in the Electromagnetic spectrum. It is a very energetic form of light that is invisible to the human eye.wavelengths above 4000A fall into the visible spectrum and are seen as the color violet. UV is separated according to wavelength into three classes : A, B' and C. The shorter the wavelength, the more energy that is carried in the light and the more dangerous it is to the human cells.
The desired wavelength range for use in nondestructive testing is between 3500 and 3800A with a peak wavelength at about 3650A. This wavelength range is used because it is in the UV-A range, which is the safest to work with. UV-B will do an effective job of causing substances to fluoresce, however, it should not be used because harmful effects such as skin burns and eye damage can occur. This wavelength of radiation is found in the arc created during the welding process. UV-C (1000 to 2800A) is even more dangerous to living cells and is used to kill bacteria in industrial and medical settings.
The desired wavelength range for use in NDT is obtained by filtering the ultraviolet light generated by the bulb. The output of a UV bulb spans a wide range of wavelengths.The short wavelength of 3120 to 3340A are produced in low levels. A peak wavelength of 3650A is produced at a very high intensity Wavelength in the visible violet range (4050A to 4350A),green- yellow (5460A), yellow (6770A) are also usually produced. The filter allows only radiation in the range of 3200 to 4000A and a little visible dark purple to pass.

Thursday, 24 November 2016

Magnetic particle inspection can be performed using particles that are highly visible under white light conditions or particles that are highly visible under ultraviolet light conditions. When an inspection is being performed using the visible color contrast particles, no special lighting is required as long as the area of inspection is well lit.A light intensity of at least 1000 lux (100fc) is recommended when visible particles are used, but a variety of light sources can be used.
When fluorescent particles are used, special ultraviolet light must be used. Fluorescence is defined as the property. of emitting radiation as a result of and during exposure to radiation. Particles used in fluorescent magnetic particle inspections are coated with a material that produces light in the visible spectrum when exposed to near - ultraviolet light. This "particle glow" provides high contrast indications on the component anywhere particles collect. Particles that fluoresce yellow - green are most common because this color matches the peak sensitivity of the human eye under dark conditions. However, particles that fluoresce red, blue, yellow, and green colors are available.

Wednesday, 23 November 2016

Multidirectional units allow the component to be magnetized in two directions, longitudinally and circumferentially, in rapid succession. Therefore, inspections are conducted without the need for a second shot in multidirectional units, the two fields are balanced so that the field strength are equal in both directions. These quickly changing balanced fields a multidirectional field in the component detection of defects lying in more than one direction.
Just as conventional wet-hirizontal systems the electrical current used in multidirectional magnetization may be Alternating half-wave direct, or full-sized. It is also possible to use a combination of currents depending on the test applications.Multidirectional magnetization can be used for a large number of production application, and high volume inspections.
To determine adequate field strength and balance of the rapidly changing fields,techniques development requires a little more effort when multirectional equipment is used. It is desirable to develop the technique using a component with known defects oriented in at least two directions, or a manufactured defect standard. Quantitative Quality Indicators (QQI) are also often used to verify the strength and direction of magnetic fields.

Tuesday, 22 November 2016

To inspect a part using a head - shot, the part is clamped between two electrical contact pads. The magnetic solution,called a bath, is then flowed over the surface of the part. The bath is then interrupted and magnetizing current is applied to the part for a short duration. typically 0.5 to 1.5 seconds.(precautions should be taken to prevent burning or overheating of the part) A circular field flowing around the circumstances of the part is created. Leakage fields from defects then attract the particles to form indications.
When the coil is used to establish a longitudinal Magnetic field within the part is placed on the inside surface of the coil. Just as done with a head shot, the bath is then flowed over the surface of the part. A magnetizing current is applied to the part for a short duration, typically 0.5 to 1.5 seconds, just after coverage with the bath is interrupted. (Precautions should be taken to prevent burning or overheating of the part) Leakage fields from defects attract the particles to form visible indications.
Theoretical horizontal unit can also be used to establish a circular magnetic field using a central conductor. This type of a setup is used to inspect parts that have an open centre, such as gears, tubes, and other ring - shaped objects. A central conductor is an electrically conductive bar that is usually made of copper or aluminium. The bar is inserted through the opening and the bar is then clamped between the contact pads When current is passed through the central conductor, a circular magnetic field flows around the bar and enters into the part or parts being inspected.

Monday, 21 November 2016

Stationary magnetic particle inspection equipment is designed for use in laboratory or production environment. The most stationary system is the wet horizontal (bench) unit. Wet horizontal units are designed to allow for batch inspections of a variety of components. The units have head and tail stocks (similar to lathe) with electrical contact that the part can be clamped between. A circular magnetic field is produced with direct magnetization. The tail stock can be moved and locked into place to accommodate parts of various lengths. To assist the operator in clamping the parts, the contact on the headstock can be moved pneumatically via a foot switch.
Most units also have a movable coil that can be moved into place so the indirect magnetization can be used to produce a longitudinal Magnetic field. Most coils have five turns and can be obtained in a variety of sizes. The wet magnetic particle solution is collected and held in a tank. A pump and hose system is used to apply the particle solution to the component being inspected. Either the visible or fluorescent particles can be used. Some of the system offer a variety of options in electrical current used for magnetizing the component. The operator has the option to use AC, half wave DC, or full wave DC.In some units, a demagnetization features is built in, which uses the coil and decaying AC.

Friday, 18 November 2016

Coils and conductive cables are used to establish a longitudinal Magnetic field within a component. When a preformed coils is used, the component is placed against the inside surface on the coil. Coils typically have three or five turns of a copper cable within the molded frame. A foot switch is often used is typically 00 extra flexible or 0000 extra flexible. The number of wraps is determined by the magnetizing force needed and of course, the length of the cable. Normally, the wraps are kept as close together as possible. When using a coil or cable wrapped into a coil, amperage is usually expressed in ampere - turns. Ampere - turns is the amperage shown on the amp meter times the number of turns in the coil.

PORTABLE POWER SUPPLIES :- Portable power supplies are used to provide the necessary electricity to the prods, coils or cables. Power supplies are commercially available in a variety of sizes. Small power supplies generally provide up to 1,500A of half - wave direct current or Alternating Current when used with 4.5meter 0000cable. They are small and light enough to be carried and operate on either 120V or 240V electrical service. when more power is necessary, mobile power supplies can be used These units also operate on 120V or 240V electrical service and can provide up to 6,000A of AC or half - wave DC when 9meters or less of 0000 cable is used

Thursday, 17 November 2016

Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current through the metal. The current passing between the prods creates a circular magnetic field around the prods that can be used in magnetic particle inspection. prods are typically made from copper and have an insulated handle to help protect the operator. One of the prods has a trigger switch so that the current can be quickly and easily turned on and off. Sometimes the two prods are connected by any insulator (as shown in the image) to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.
If proper contact is not maintained between the prods and the component surface. electrical arcing can occur and cause damage to the component. For this reason, the use of prods are not allowed when inspecting aerospace and other critical components. To help prevent arcing, the prod tips should be inspected frequently to ensure that they are not oxidized, covered with scale or other contaminant,or damaged.

Wednesday, 16 November 2016

Today, most of the equipment used to create the magnetic field used in MPI is based on electromagnetism. That's, using an electrical current to produce the magnetic field. An Electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field.

It is basically made by wrapping an electrical coil around a piece of soft ferromagnetic steel. A switch is included in the electrical circuit so that the current and, therefore, the magnetic field can be turned on and off. They can be powered with Alternating Current from a wall socket or by direct current from a battery pack. This type of magnet generates a very strong magnetic field in a local area where the poles of the magnet touch the part being inspected. Some yokes can lift weights in excess of 40 pounds.